feat(schema): Support python @context_func for jinja templates (#6729)

Author: ovr

packages/cubejs-backend-native/TECH.md

@@ -0,0 +1,38 @@
+### Python load `cube.py`:
+
+```mermaid
+sequenceDiagram
+    participant Node
+    participant Native
+    participant PythonRuntime
+    participant Python
+
+    Node->>Native: pythonLoadConfig
+    activate Node
+    Native->>+Python: loading module
+    Python-->>+Node: CubePyConfig
+    deactivate Node
+```
+
+# Calling python
+
+```py
+def context_to_app_id(ctx):
+    print('content_to_app_id', ctx)
+
+    return 'CUBEJS_APP_{}'.format(ctx.securityContext.userId)
+```
+
+```mermaid
+sequenceDiagram
+    participant Node
+    participant Native
+    participant PythonRuntime
+    participant Python
+
+    Node->>Native: config.contextToAppId
+    activate Node
+    Native->>+Python: scheduling call for context_to_app_id (Py<PyFunction.)
+    Python-->>+Node: defer
+    deactivate Node
+```